As with the theme of Hardmodes, Rotface is mostly the same but hits a lot harder. In fact, rotface is one of the easier hard modes to achieve.
There are 3 real mechanic changes, first is that the infection causes a much higher healing debuff, so it is recommended that the person is dispelled as soon as possible and healed until they merge their ooze.
The second is the pools of goo coming out of the pipes now cause a stacking slow debuff that you need to be wary of. Most especially the tank who is doing the kiting. It is recommended you either skirt the edges of it, or use freedoms and other snare removing abilities to get the stacks off as soon as possible. If you are kiting a small ooze to the big ooze, it is wise to not even attempt to go into the green patches, just wait for the big ooze to get to you.
Third is rotface will cast a Vile Gas type thing on anyone at range. The off tank is a viable target for this as well. The vile gas will disorient you if you move. So, get all your range and healers out of melee and spread out around the room as best they can to reduce the risk of the off tank getting it.
Other than that, it is business as usual, just with more hitpoints and everything hits harder!